1. Field of the Invention
This invention relates to the manufacture of ceramic dual-in-line packages and more particularly to preparing elements of dual-in-line packages having a layer of sealing glass fused to the surface.
2. Description of the Prior Art
Integrated circuits, formed on a small chip of silicon, are often enclosed in a dual-in-line package which is a rectangular enclosure having electrically conducting pins extending from two parallel sides of the package for making electrical connections to the enclosed integrated circuit. When such packages are made of ceramic they comprise a ceramic substrate (for example, about 7 mm wide.times.20 mm long.times.1.9 mm thick) having a depression in the center of one of the large flat surfaces. The bottom of the depression is usually coated with gold to serve as an electrical contact. The silicon chip containing the integrated circuit is bonded to the bottom of the depression. The lead pins are then located along the long sides of the substrate in a lead frame and are connected to the integrated circuit. A ceramic cover, having about the same length and width as the substrate but a smaller thickness, e.g., about 1.3 mm, and optionally also having a central depression, is then placed on top of the substrate. Either the substrate and/or the cover is provided with a layer of low-melting sealing glass disposed generally around the edges of a mating face. After assembly the package is heated to a temperature at which the sealing glass softens and forms a bond and seal between the cover and the substrate.
Hitherto, the layer of sealing glass on the substrate or cover has been formed by a method involving screen printing. According to the screen printing process, a paste comprising a mixture of powdered low-melting sealing glass and a binder, such as a synthetic resin dissolved in a suitable solvent, is applied to the substrate and/or cover by screen printing and dried. The steps of application and drying are repeated several times in order to build up the layer to the proper thickness (0.35-0.5 mm). The layer is then fixed by baking, i.e., by raising the temperature of the coated element to about 410.degree. C. over a period of about 1.5 hours.
This method of preparing the element having a layer of sealing glass is very inefficient. It requires three to six screen printing and drying operations to build up the required film thickness of 0.35-0.5 mm. Furthermore, since the printed film shrinks about 30% when it is baked, the printed layer must be made oversize to account for the shrinkage. Since the printed paste thus extends over the sides of the substrate, the process has the further drawback that the sealing glass may adhere to the sides of the substrate or damage the gold plating of the central depression where the silicon chip is mounted. In addition, proper registration of the successive printing steps is difficult to attain and hence it is difficult to obtain glass layers of exact dimensions, even with careful work.
Another problem with the printing technique is the frequent presence of air bubbles which are formed in the glass layer and cause pinholes which prevent the seal from being perfectly gas-tight.
Therefore, a need has continued to exist for a method of providing an element of a dual-in-line package having a layer of sealing glass and therefore being ready for assembly which overcomes the deficiencies of the previously known methods.